As explained in detail in U.S. Pat. Nos. 4,501,943 and 4,523,068, which are hereby incorporated by reference, in the manufacture of lead acid storage batteries, it is customary to assemble the individual battery cells into a case with the terminal cells each having an upstanding lead terminal post located at opposed ends of the battery. It is also customary to then position a cover having cylindrical lead bushings fixed therein onto the battery case with the terminal posts extending through respective ones of the bushings, and to thereafter heat, melt, fuse, mold, cool, and freeze the upper ends of the bushings and posts to form the finished battery terminals. Since the case and cover in modern day batteries are commonly made of plastic, care must be taken in fusing the terminal posts and bushings so as not to melt or otherwise damage the immediately adjacent portions of the cover, which can either render the battery defective or sufficiently weaken the seal and support between the cover and bushings as to create leaks or other potentially dangerous conditions during use of the battery.
Prior to the inventions of the '943 and '068 Patents, it was common practice to fuse the terminal posts and bushings by melting the ends thereof by means of an acetylene torch which was manually held and operated. Not only did this procedure fail to lend itself to use in a fully automated battery production line, but the quality and depth of the fused areas of the terminal posts and bushings varied with the operator who performed the fusion process, and even between terminals of successive batteries fused by the same operator since there was no reliable means for controlling the degree of fusion that was effected. Moreover, it was not easily determinable whether minimum required fusion depths were obtained, i.e., generally considered to be between 1/8 and 3/16 inch as measured from the top of the finished terminal.
While, prior to the inventions of the '943 and '068 Patents, various proposals had been made for automatically fusing battery terminal posts and cover bushings by means of either acetylene torch heating, electrical resistance heating, or electrical induction heating, such proposals had all faced various drawbacks. For example, these proposals suffered from drawbacks such as the inability to obtain reliable fusion depths within the requisite processing time, undesirable melting of the cover about the bushings, and unacceptable appearance of the finished terminals. Since lead oxides contained within the lead bushings tend to float to the surface during melting, under some circumstances unsightly irregularities in the surfaces of the finished battery terminals had resulted.
The inventions of the '943 and '068 Patents overcame these problems by providing an induction heating apparatus and method for automatically fusing battery terminals. In practicing the inventions of the '943 and '068 Patents, high-power RF induction heating generators have typically been employed. These generators have often utilized a three-phase SCR power control circuit to control the power supplied to the induction coils by the generator. In practice, however, the SCR control circuits of these prior art generators have been prone to failure. Indeed, approximately 80%-90% of the downtime associated with the induction heating generators employing these SCR control circuits is attributable to failures in those SCR control circuits. Such failures have resulted in increased maintenance costs in terms of personnel time and additional economic losses in terms of decreased production due to machine downtime. Other drawbacks of these SCR control circuits include their cost (typically on the order of $3,500.00 per unit) and the inherent difficulty involved in troubleshooting these complicated devices.
On occasion, three phase thyratron tube networks have been used instead of SCR control circuits to control the power supplied to the work coils of the generator. However, these thyratron tube networks have suffered drawbacks similar to those described above. Specifically, such thyratron tube networks are expensive; are prone to failure; are difficult to troubleshoot; and require high maintenance.
Another prior art alternative to using SCR control circuits in induction heating generators is effected by switching the power supplied to the induction coils between fully on and fully off by selectively opening and closing the large, three phase, mechanical plate contactor provided in the induction heating generator. Although, due to its simplicity, this approach is relatively inexpensive and easy to troubleshoot, it suffers from certain other drawbacks. For example, since in order to implement this alternative the plate contactor must be opened and closed under full load, the plate contactor experiences considerable stress and wear, thus, requiring a high degree of maintenance. More significantly, as taught by U.S. Pat. Nos. 9,501,943 and 4,523,068, when fusing battery terminals it is advantageous to provide a controlled de-energization of the induction heating coils over a predetermined time period in order to obtain a good surface appearance of the completed battery terminals. In such instances, it is also advantageous to provide a controlled energization of those coils over a predetermined time period in the form of a relatively linear ramped-up or stepped-up current supply. However, opening and closing the plate contactor to regulate power cannot provide either a controlled energization or a controlled de-energization. On the contrary, it has only two power states, namely, fully on and fully off, and the transitions therebetween are abrupt.